The invention relates to a radiation detector for converting electromagnetic radiation into electric charge carriers. The invention also relates to an X-ray examination apparatus provided with radiation detector of this kind and to a method of manufacturing a radiation detector.
Radiation detectors are used notably in the medical field, that is, for X-ray examinations, and serve to form radiation images of an object to be examined, usually a patient, in the context of mostly a medical examination or therapy. An image pick-up system which also includes the X-ray detector is used to form images of the object to be examined which is exposed to the X-rays, said images being output, for example, via a monitor. The X-rays incident on the X-ray detector are converted into electric charge carriers in a converter arrangement. The electric charge carriers generated in the converter arrangement are collected in associated capacitances so as to be read out by a downstream electronic read-out circuit arrangement.
Generally speaking, a radiation detector is constructed in such a manner that the electromagnetic radiation is incident on a converter arrangement. Depending on the specific construction of the radiation detector, either a directly converting converter layer in the converter arrangement converts the radiation into electric charge carriers which are subsequently read out, or the radiation is first converted into visible light by means of two converter layers and subsequently, that is, in a second converter layer which is arranged therebelow, notably a photosensor arrangement, into electric charge carriers so as to be read out.
In the case of radiation detectors provided with a converter arrangement which includes two converter layers, the first converter layer is provided as a scintillator layer of, for example, CsI:Ti. Underneath this first converter layer, that is, viewed in the direction of the incident radiation, the second converter layer is formed as a photosensor arrangement.
The individual photosensors detect the radiation converted into visible light, said radiation then being read out one pixel after the other via the individual photosensors. The conversion of radiation directly into electric charge carriers in radiation detectors provided with a converter arrangement with only a single converter layer is also referred to as direct conversion. The first converter layer is then constructed as a directly converting semiconductor layer of, for example, amorphous selenium. Radiation detectors with direct conversion in only a single converter layer may also be realized by means of a PbO layer, the charge carriers produced then being stored and subsequently read out.
Underneath the converter layer or layers (depending on the construction of the radiation detector) there is provided an illumination device which serves to reset the individual pixels of the photosensor arrangement in the context of the preparation of the radiation detector for a further exposure. For radiation detectors which include only a single converter layer for direct conversion it is also effective to induce a charge carrier flood by way of a reset light pulse, thus exerting a positive effect on the decay behavior of the converter layer so as to enable a faster series of X-ray images and/or a better quality to be achieved without image artifacts.
The converter layers mentioned thus far are supported by a substrate of, for example, glass.
It has been found that the photosensor arrangement or second converter layer exhibits a slow decay which has an adverse effect on successive image exposures. Such a decay behavior is detrimental notably when many images are acquired per unit of time. The cause of such decay lies in physical processes which take place in the photosensors upon incidence of optical photons. When a photon is incident on the semiconductor material of the photosensor arrangement, an electron is moved from the valence band to the conduction band and the electric charge thus produced is stored on electrodes of the semiconductor layer which constitute a capacitance. However, because so-called traps occur in the semiconductor layer of the photosensor arrangement due to contaminations and grid defects, many electrons remain behind in the semiconductor layer. Normally speaking the charge carriers present in the traps are thermally emitted in the course of time and transferred to the electrodes, be it that this may take a long period of time. Because of this quasi-thermal emission, which also takes place when the photosensor arrangement has already been read out and a second image is formed, so-called afterimages or image artifacts of the previously acquired image will be visible in subsequently acquired images.
In order to solve this problem, it is known to read out the photosensor arrangement after successful formation of an X-ray image and to make the illumination device deliver subsequently at least one light pulse which acts on the second converter layer. The light pulse floods the second converter layer with charge carriers and the traps in all pixels are uniformly occupied. In order to achieve an as effective as possible occupation of the traps by charge carriers, the illumination device emits light of a given wavelength in the form of one or more separate light pulses in rapid succession. For effective resetting, however, it is a prerequisite that the light emitted by the illumination device is uniformly distributed in the direction of the photosensor arrangement.
DE 199 14 217 describes an X-ray detector in which a scintillator arrangement is arranged over a pixel matrix, both elements being arranged over a glass support which supports the scintillator arrangement and the pixel matrix. Underneath the glass support there is provided a layer of air and the light source or illumination device is situated underneath said layer of air. This layer of air is necessary to achieve a spatial distribution of the light emitted by the illumination device and to distribute the light as homogeneously as possible. Direct arrangement of the illumination device underneath the glass support, that is, without a corresponding layer of air, is detrimental because in that case the required homogeneous light distribution will not be achieved so that the resetting of the photosensor arrangement and also of the scintillator or converter layer is not effective. The glass support, serving notably for stabilizing the photosensor arrangement and the scintillator or converter layer carried by the support, cannot realize such a homogeneous light distribution. A further drawback of such a detector resides in its considerable height which is due to the presence of the glass support layer and the layer of air.